This invention relates to an optical servo-positioning system for positioning the read-out beam of an optical read-out system which reads out pre-recorded digital information from a recording medium such as a disc.
There has been a trend toward the development of digital recording and playback techniques. Video and audio signals previously recorded only in an analogue form, such as in conventional audio records or video tapes, are now being recorded in digital form on discs. Digital recording systems convert analogue information, such as, for example, an audio signal into digital information and then record that digital information onto a disc as "pits" forming a circular or spiral track of recorded information. The recorded information is read from a previously recorded disc by irradiating the spiral track with a read-out beam of convergent light, such as from a laser and then detecting the variation of a beam reflected from the disc.
In such optical digital information recording and read-out systems, it is an absolute necessity that the read-out beam be always accurately positioned over the data track being read so as to accurately sense the information recorded on the disc. This is sometimes referred to as maintaining the registration of the read-out beam.
Deviation of the incident read-out beam from the center of the track may cause the output of the reflected beam to be distorted or at too low an intensity to be sensed. As a result, the previously recorded information may not accurately read out. To prevent deviation or misregistration of the read-out beam, optical information read-out systems are usually provided with an optical servo-positioning system for positioning the spot of incidence of the read-out beam on the data track of the disc.
U.S. Pat. No. 4,118,735--Wilkinson--discloses a known optical servo-positioning system for positioning a read-out beam on a spiral data track. The system includes an articulated mirror for controlling the position of the light beam spot on the disc. An oscillator, which generates a low-frequency signal, drives the articulated mirror to wobble (dither) the spot so as to traverse the data track from one side of the track to the other with a very small lateral excursion. A photocell is positioned so as to detect a beam of light reflected from the data track. An output signal of the photocell includes error magnitude and direction information for the read-out beam. This magnitude and direction information is determined as a function of the phase relationship between the reflected light beam and the drive signal to the articulate mirror. If the beam is accurately centered, the intensity signal from the photocell is at a minimum. If the reflected beam is to the right of center of the data track, the intensity signal from the photocell increases, and, when multiplied by the driving signal, produces a product that has, for example, a positive value. However, if the reflected beam is to the left, the product of the intensity signal from the photocell and the driving signal has an opposite or negative value. Therefore, the direction of correction required is represented by the polarity of the multiplied signal while the amount of correction is represented by the articulate mirror.
However, known systems such as the Wilkinson system, discussed above, have the following problem. The wobbling frequency must be selected so as not to interfere with the information recorded on the disc. If the information recorded on the disc is a video signal which has the frequency spectrum of 1 to 5 MHz, the wobbling frequency of the servo-positioning system can be selected so as to be out of the video signal frequency spectrum band that is recorded, for example several 10 kHz. However, when the digital information recorded on the disc has an extended frequency spectrum range, such as from zero to several MHz, it is difficult to separate the wobbling signal from the pre-recorded signal so that they do not interfere with one another. Various modulation schemes such as NRZ (Non-Return to Zero), MFM (Modified Frequency Modulation), PM (Phase Modulation) and others are employed to convert an analogue signal into digital form for recording on a disc. These modulation systems have spectrum components in the low frequency band. Using known servo-positioning arrangements, a wobbling signal will interfere with these low frequency components. To prevent such interference, it would be necessary to degrade system performance by eliminating the low frequency components of the modulated digital data recorded on the disc.